This invention relates to an apparatus and method for spatially coupling an electro-magnet to a support armature such that movement of either the electro-magnetic or support armature is automatically compensated for so that such movement is not substantially transferred between the electro-magnet or the support armature. Such apparatus are often referred to as support actuators.
At present, it is possible to couple a base to a vibrating load support armature using an electro-magnet mounted to the base, the electro-magnet when energised with a suitable fixed current produces a magnetic field which exerts an attraction force on the support armature so as to attract the support armature towards the electro-magnet when the support armature is positioned within the magnetic field. It should be understood that there is a physical gap between the support armature and the electro-magnet.
An electro-magnet can be considered as a dual input, single output device. The output being the attraction force exerted by the magnetic field on the support armature. The attraction force is dependent on the inputs, the first being a current which is used to energise the electro-magnet and the second being dependent on the magnitude of the gap. For example, the attraction force exerted on the support armature increases as the support armature is positioned closer to the electro-magnet, e.g. the magnitude of the gap is reduced, and decreases as it is positioned further from the electro-magnet, e.g. the magnitude of the gap is increased. The attraction force exerted by an electro-magnet on a support armature is analogous to a strong negative spring.
It is common practice to employ high gain feedback control on the electro-magnet in an attempt to convert the strong negative spring into a weak negative spring so that more precise control of the attraction force of the electro-magnet can be applied.
However, such high gain feedback cannot be maintained over an infinite bandwidth and in practice the gain of the feedback must ultimately roll-off towards zero with increasing frequency. The result of the roll off effect causes a change in phase and the weak negative spring then tends to behave more like a negative damper. This feeds energy into variations in the gap which can readily excite any structural resonance in the roll off frequency range. This in turn generates undesirable force variations on the support armature.
It is the object of this invention to obviate or mitigate this disadvantage.
According to a first aspect of the present invention there is provided an apparatus, comprising
a current controller operable to produce a variable current,
an electro-magnet connected to receive said variable current and to generate a magnetic field having a flux intensity dependant on said variable current,
a support armature separated from said electro-magnet by an operational gap and supported by said magnetic field,
a first control means arranged to detect variation in said operational gap and to operate said current controller dependant on variation in said operational gap,
a second control means arranged to detect variation of said flux intensity and to operate said current controller dependant on variation of said flux intensity, and
said first control means and said second control means being arranged to vary said current in said electro-magnet whereby the flux intensity of said magnetic field will maintain said operational gap substantially constant.
Preferably, said first control means may comprise a gap sensor arranged to detect variation in said operational gap, said first control means may also be arranged to generated a control signal dependant on variation in said operational gap, and said control signal may be arranged to operate said current controller. The said first control means may be arranged in a feedforward path from said gap sensor to said current controller.
Preferably, said second control means may comprise a flux sensor arranged to detect said variation in said flux intensity, said second control means may also be arranged to generate a control signal dependant on variation in said flux intensity, and said control signal may be arranged to operate said current controller. The said second control means may be arranged in a feedback path from said flux sensor to said current controller.
Operation of said controller may be determined by a transfer function of a relationship between said control signal generated by said first control means, said control signal generated by said second control means and said variable current applied to said electro-magnet, and said current controller may be arranged to produce said variable current dependant on said transfer function.
According to a second aspect of the present invention there is provided an apparatus for isolating vibration between a load and a support structure, comprising
a current controller operable to produce a variable current,
an electro-magnet connected to receive said variable current and to generate a magnetic field having a flux intensity dependent on said variable current,
said load supported from said support structure by said magnetic field between said electro-magnet and said support armature,
a first control means arranged to detect variations in said operational gap and to operate said current controller dependant on variations in said operation gap,
a second control means arranged to detect variations of said flux intensity and to operate said current controller dependant on variations of said flux intensity, and
said first control means and said second control means being arranged to vary said current in said electro-magnet whereby the flux intensity of said magnetic field will maintain said operational gap substantially constant and inhibit transmission of vibration therebetween.
According to a third aspect of the present invention there is provided a method of spatially coupling a support armature with respect to an electro-magnet, comprising
applying a current to said electro-magnet to generate a magnetic field having a flux intensity dependant on said current,
and controlling an operational gap between said armature and said electro-magnet by detecting variation in said operational gap and detecting variation in said flux intensity, and varying said current dependant on the variation in both said operational gap and said flux intensity.
Preferably, the method may also comprise varying said current dependant on a transfer function of a relationship between
a. variations in said operational gap,
b. variations in said flux intensity, and
c. said current applied to said electro-magnet
According to a fourth aspect of the present invention there is provided a method of isolating vibration between a load and a support structure, comprising
supporting a load from said support structure by a magnetic field across an operation gap between an armature and an electro-magnet,
and controlling said operational gap by detecting variation in said operational gap and detecting variations in said flux intensity, and varying flux intensity of said magnetic field dependant on the variation both in said operational gap and said flux intensity.
The positioning and protection of these electro-magnets and support armatures is taught in our co-pending British Applications GB. A [that is British Application 9604973.9] and GB A [that is British Application 9604952.3] both filed on Mar. 8, 1996, the whole contents of each Application being incorporated herein by reference.